Least-favorite? Oh, that's hard... I pretty much like most of the songs in the Zelda games.
Oh, but if I were to choose, I'd probably say the song that plays in the Illusionary Room in Ganon's Tower for the Wind Waker. I don't know... it just doesn't have much excitement to it. And it kind of sounds like a really poorly-made music box that just reached the end of its winding, and is beginning to slow down and be all boring. Well, it does!